The sport known by the name of "body-boarding" consists in trying to move on the crest or close to the crest of a wave, while remaining face-down on a board whose dimensions are generally less than those of boards used for surfing, which is performed standing on the board.
When waiting for a wave, a sportsperson who is "body-boarding" adopts a position which is favourable for him/her to be carried along by the crest of the wave. When the crest arrives, it is furthermore necessary for him/her to exert sufficient energy to gain a speed, in the same direction as the crest, which allows the phenomenon of being carried along by the wave. For this purpose, the sportsperson is generally equipped with fins which allow him/her to impart sufficient change in momentum when the crest of the wave arrives. For these operations, the fin must remain substantially at the surface of the water and furthermore contribute to steering the sportsperson and his/her board.
This explanation demonstrates why the fin must be made of a material which is sufficiently hard to allow it to strike the water efficiently and obtain the desired momentum.
It is also apparent that abrupt movements are involved and that it is important for the fin to be fastened reliably to the foot of the user.
In the case of fins intended for diving, the forces transmitted to the fin are less intense, but it is still necessary for the fin to be relatively hard in order to retain a good degree of efficiency; similarly for aerobics in water, it is necessary to exert a force which will cause energy to be expended.
When engaging in the water sports mentioned hereinabove, the sportsperson is moreover frequently called upon to walk wearing fins on his/her feet between two sporting sessions. This results in sand, or other foreign bodies, being capable of entering the fins during such a walk, which is irritating.